Wagner - Ride of the Valkyries
Advice
This is one of those staple orchestral excerpts for trombone- one of the most famous of all time. Aside from the obvious "must-haves" like tone quality, intonation, etc., this one is really all about the rhythm. Listen to all recordings on repeat until you get the correct idea in your head. The video with Mr. Alessi has a great explanation of common rhythmic issues that players encounter and how to avoid them. Notes should be full value and longer than you think to fit the orchestral style. I recommend recording yourself and listening back to find areas for improvement.
Resources
Full Orchestra Recording - The Wicked North
Masterclass Breakdown with Thomas Leyendecker - Carnegie Hall
Excerpt Breakdown with Joe Alessi
Low Brass Section Recording - STS
Bass Trombone Only Recording - Christian Jones
Wagner - Das Rheingold
Advice
This is an absolute staple for bass trombone auditions. I don't think I've ever done an orchestral bass trombone audition where this excerpt wasn't requested. The main thing they are judging/looking for is your sound in the low register. Make sure to listen to recordings for the full piece to know how this should sound dynamically and stylistically. Learning how to gain the control to play loudly with a full sound and good tone in this low register is a process that requires lots of practice. Also, practice with a 16th note pulse metronome to make sure your dotted eigth and sixteenth note rhythms don't sound lazy. Lots of recording yourself and listening back on this one.
Resources
Full Orchestra Recording - The Wicked North